Frequency determining unit



Sept- 21, 1954 F. RIEBER FREQUENCY DETERMINING UNIT 3 Sheets-Sheet 1 Filed March 28, 1946 Sept. 2l, 1954 F. Feu-:BER 2,689,943

FREQUENCY DETERMINING UNIT Filed March 28, 1946 3 Sheets-Sheet 2 Fig. Z

INVENTOR.

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Sept. 21, 1954 F RIEBER 2,689,943

` FREQUENCY DETERMINING UNIT Filed March 28, 1946 3 Sheets-Sheet 3 Figs T W1 I@ Patented Sept. 21, 1954 UNITED STATES PATfEN'T GFFI CE 'FREQUENCY kDETERMININ-Gr UNIT Frank RiehenJNew Yon-k, N. Y., assigner, by mesne assignments, l'to Frank sxstreeter, trus'tcclor The Vihro'troni Company Ltd., cWYork, N. X.,

aicopartnership Application March 28, 1946,`SeIil'N0. (557;880

(Cl. S33-71) This 'invention relates to frequency-'determining units which are .of general application, but more particularlylto such devices of the-type which mayA beprecisely adjusted toany Vselected frequency `within a .widerangebf frequencies.

vWhen regulatable control .of frequency ,is required, resort hasheretofore -been "had to bal- .anced electricinductance andcapacity elements, oneor both vof which is regulatable. .Such devices,`however, fail 'to provide an accurate and permanent ,control of .frequency at 'the value tto which it maybe regulated. 'They .are .incapable ofsharp tuning and subject to drift. Where accurate control of [frequency has been required, y.resort has heretofore been had to the use of crystals. By the use of Va crystal, sharp and accurate control of frequency may be obtained, .but `only at .the resonantfrequency of .the crystal, which is invariable, There has thus been no ,frequency controllingmeans which is .both sharp .and accurate and .alsoregulatable Such .means 4are -needed ,in ltelemetering and for other pur- .poses.

vIt is an object of .the.invention, therefore, to lprovide anew and` improvedifrequency-determin- Ting unit .which avoids one or more o'f ,the above- .mentioned limitations and disadvantages .of the prior art arrangements.

It is another object-oi the invention to pro- .videa .new and improved frequency-.determining unit .which is ,characterized iby one or more of the following .advantageous characteristics: an accurately .reproducible "functional relation be- .tween adjustment .and frequency; an extremely .sharply selective .and accurate tuning characteristic; .a high degree .of stabilitywith respect to .time and .with .respect to variations in lthe .characteristics of .elements'with whichfit'is associated `.to .ma-ke Vvup .a system, and unusually .low losses, `thatis, .a .high Q which contributes to its sharp selectivity.

It .is afurther Aobject of the invention to pro- .videa new and Vimproved electro-.mechanical oscillation .generator which is .particularly Vadapted to incorporate the frequency-determining unit of the invention.

A device embodying the invention in the form which I now consider most desirableprovi'des :a .simple means for determining frequency `which is both sharp and accurate 'and `attheSametime regulatable over a wide `range. It includes an adjustable mechanical'vibratory'element and two electric circuits, "one of which supplies energy 'to the mechanical element 'to -set it into lvibration andthe `other of which `receives Aenergy from the Avibration of the mechanical element. vIhemevclla'nical element thus provides a link @between fthe -two electricicircuits Whichlpermits the transier of energy :from :one circuit to 'the `other :only :at vthe frequency-of vibration .to vwhich .the unechanicalelement is adjusted. kIt thusconstitutes a filter and llike'other-electrical lters it may be lused in the feedback circuit of an amplier to provide an oscillator.

The 'mechanical vibratory lelement is a stretched conductive strand mounted in such manner that its `tension may be'adjusted over a wide lrange and when adjusted `will not Abe 'changed either Iby linternalchanges in the strand or its 4mounting or by `changes invexternal con- "ditions such as temperature. The strand `is lplaced 'in 'a 'constant ymagnetic field, such as that produced by a j'permanent magnet, and Ais set intovibration byjpassing through it lanalternat Ving current `whose Iamplitude is limited 'by an automatic volume control. The means for avoid- "ing changes -in the'ad'justedtension or the strand and Athe `-means for 'limiting the amplitude of Athe applied current 'and 4consequently -`the amplitude `of vibration kof the strand give `the strand a dennite vibration frequency Ifor -each adjusted ten- -son `of the Astrand and insure `the maintenance of this frequency after adjustment and the repetition of this frequency ywhen the Asame adjustment is made again.

Tn 4order to make the device sharply tunable, radiation 'of -energy Cfrom the vibrating strand is limited. Substantial mechanical radiation of energy into `the parts 'supporting vthe lstrand is 'eliminated l"by providing 'for setting the strand into Vibration by means of equal 'and opposite forces on equal parts of 'the strand. Loss of energy by electrical radiation 'is limited "by 4makingthe impedance 'of the network connecting the two ends of the strand as great or greater "than the static resistance ofthe strand.

`While Ivarious Amethods of utilizing the vibrating strand as va link between ltwo `electrical circuits :may be used, the arrangement ywhich Toon- `sider mostvdesirable lconsists in 4incorporating Athe vibrating `strand yas `part `'of one-element of 'a bal-- :anced `.network V-so :ar-ranged ithat no current is transmitted from the `input circuit to the output circuit 'except when .the :strand 4is 2in ivibration. .At 'the -ifrequencyrof vibration yof .the strand, :the idyna-mic impedance of 4the strand yunbalances the Acircuit cand .permit-s the `transfer vof energy to the output circuit. .An .accurate :balancing of the static .resistance of. the vibratory .strand is obtained by .including in .the branch .of the vcircuit .adjacent to vthat .containing the vibratory strand a resistor consisting of another .strand of the same material and ldimensions as 'the vibratory strand. 'The resistor strand is 'most desirably 'at the same temperature and under `the "same "tension as the vibratory strand in 4order that it -may lhave precisely -the same static resistance. The @resistor vstrand creates no dynamic impedance I'as it :is out-side the .magnetic l'field in which `.the 'vibratory strand sis located.

The invention will be further explained in connection with specific embodiments of it `which are shown in the accompanying drawings, in which:

Fig. l is a perspective view of the mechanical element of the system showing the casing in vertical section and with portions of the framework and casing broken away;

Fig. 1A is a transverse section of one of the clamps holding the ends of the vibratory strand;

Fig. 2 is a side View of the vibratory strand and the magnets with a portion of the magnets broken away showing positions assumed by the strand in vibration;

Fig. 3 is a fragmentary perspective View of the mechanical element showing a modiiied form in which a double vibratory strand is used;

Figs. 4, 5 and 6 are wiring diagrams of filters embodying the invention; and

Fig. 'I is a wiring diagram of an oscillator embodying the invention.

'I'he mechanical element of the device is shown in Fig. 1. The vibratory strand I is stretched between two clamps II, I2. The lower clamp I2 is insulated and secured to a block I3 iixed on the lower end of a rod I4 which is suspended from a forwardly projecting arm I5 at the top of a rigid framework I6. The upper clamp II is secured at the lower end of a bar II attached to a lever I8 pivoted on a rlexible hinge strip I9 mounted on the arm I5 of the frame.

The lower end of the bar I'I is secured against horizontal movement by attachment to a collar 22 loosely surrounding the rod I4 and secured to an arm 2I mounted on the rod I4 by means of a flat piece 23 which is suiiiciently ileXible to permit slight upward and downward movement of the clamp II.

The tension of the strand I0 is regulated by a graduated hand wheel 25 mounted on a shaft 26 which extends through the frame I6 and has, near its upper end, a micrometer thread to engage a female thread 2'! at the top of the frame. The upper end of the shaft 26 bears against the lever I8, which is held in contact with it by a tension spring 28. It is apparent that turning of the hand wheel will tip the lever I8 and raise or lower the bar I"I and the clamp II so as to in crease or decrease the tension on the vibratory strand I0.

To avoid a change in the tension of the strand after its tension has been adjusted by the wheel 25, the strand I0 and the parts receiving the stresses resulting from the tension of the strand are made of material free from local strains so that they will not change their dimensions with lapse of time. The strand is made of material which is free from local strains and has no tendency to creep under tension. The parts which are placed under stress by the tension of the strand are the clamps II, I2, the rod I4 which is put under a compressive stress, the hinge strip I9 which is also under a compressive stress, the bar II which is put under a tension stress, the lever I8 which is under a bending stress, the arm I5 of the frame, the projecting upper end of the threaded shaft 26, and the spring 2B which exerts a tension much greater than that of the strand. These parts collectively (with the exception of the spring 28 which may be regarded merely as a connection between the lever and the adjusting shaft) constitute an adjustable stretcher for the strand I0. After the stretcher is assembled, all its parts are subjected to thermal cycling, that 4 is, to alternate heating and cooling, until they are entirely free from local strains.

To prevent changes in temperature from changing the tension of the strand, the strand II) and the parts of the stretcher which determine the distance between the clamps at the two ends of the strand are thermally balanced. This result may easily be obtained by a proper choice of materials because of the fact that the stretcher for the strand is of composite construction. Thermal expansion of the rod I4 and of fulcrum strip I9 tends to increase the tension on the strand, while thermal expansion of the bar II tends to reduce the tension on the strand. A thermal balance is obtained by selecting the materials for these parts in such a way that the thermal coeiiicient of expansion of the rod I4 multiplied by its length plus the thermal coefcient of expansion of the fulcrum strip I9 multiplied by its height minus the thermal coeiiicient of expansion of the bar I'I multiplied by its length is equal to the thermal coefficient of ex pansion of the strand I0 multiplied by its length. This may best be done by selecting for the strand I0 a material of low coefficient of thermal expansion and for the rod I4 a material whose coefficient of expansion is not Very much greater than that of the strand I0. The thermal balancing is completed by making the projecting upper end of the shaft 26 and the fulcrum strip I9 of the same length and of the same material, or of materials of the same thermal coefficient of expansion, so that their expansion or contraction with changes in temperature causes no tilting of the lever I8.

The best material which I have found for the strand I0 is cold drawn tungsten. A wire of this material is free from local strains and is not subject to creeping under tension. The tension on such a wire may be brought nearly to the value which ruptures the wire without causing any perceptible creeping. I have found that a tungsten wire of a length of 21A; inches and a diameter of 0.0006 inch, when subjected to a tension of about 10 grams, has a natural period of vibration, in its first mode, of 1200 cycles per second. By stretching such a wire 1/100 of an inchl to a length of 2.51 inches, the tension on the wire is increased to a little over 40 grams and the natural frequency of vibration is brought up to 2400 cycles per second. Thus, by using such a wire of these dimensions in the device described, a movement of only 1/100 of an inch on the part of the upper clamp is required to double the natural frequency of vibration. The micrometer shaft 21 thus provides for continuous adjustment of frequency over a wide range.

A further advantage of using cold drawn tungsten for the strand I0 is that this material has a very low thermal coefficient of expansion. When it is used for the strand IU, the rod I4 may desirably be made of glass-bonded mica. This materia1 has low mechanical hysteresis, is normally free from local strains, and has a thermal coeiiicient of expansion somewhat greater than that of cold drawn tungsten. A thermal balance may then be obtained by making the bar I'I 0f aluminum or some other material having a coefiicient of expansion greater than that of rod I4.

It will be seen that the mounting which has been described avoids changes in the tension on the strand II] with changes in temperature, provided that the strand and the parts supporting it are at the same temperature. A uniform distribution of temperature to the different parts agregame of the=device ismaintainedi notwithstanding rapid changesin-loutside! temperature by enclosing the strand and its mounting in valcasingswhosewall has anouter? layer 130 J of heat? conductingmaterial such as metal, an intermediate f layer 3 l of ...heat insulation, and an Ainner .layer ..32 l of :heat conducting.m'aterial. The insulation 1.3I does not, and is not intended to,-.maintainoaiconstant;tem perature within vthe casing, but.it,. inicombination with the vtwo.conducting layers, :serves tokeep thextemperature substantially uniform within ithe casingsandto preventrapid changes in'itemperature within the casing. .Thisfavoidsnnequaltem perature rises `.in .different elements of thezrunit which would. votherwise occurwith: rapid' temperaturel changes "due to the .different thermaljnertias oftheielements andresults in keepingtheistrand maudits mounting always at thelsameitemperature soxthat i the ,compensating means .abovedescribedareeffective toprevent changes inpexternal :temperature vfrom changing 2the *tension `on th'e strand In .order to :reduce or Ieliminate acousti'czradiationaV from thevibratory system, the atmospheric pressure within:theycontainer,rc0m prising the elements 30, 3l, and 32, kmay bereducedasbywholly or partially evacuating-:the container.

(In orderto setthe strand Il) into'vibrationa-an alternating .electric current is passed through it fromrend vto'iend and a magneticiluxisipassed across'it. The uxdensity across the strand is rnadeihigh,` for example, 26,000 lines vper square centimeter, .in order that a current of low amperageimay be used. The :strand It). is most desirably made `of non-magnetic material, for if .itvwere ma'degof. magnetic material the rreactionl between it and the magnetsproducingfthe flux wouldtend to draw it .at right angles tothe direction .in whichiit is Aurged by .the reaction betweenthe electric'current andthe magnetic flux and would tend to makeits vibration irregular, andmight evenpbring it into contact with the magnets and prevent it-from vibrating when .theilux Adensity isfhigh.

:In order` to setthe strand l Il into vibration and at :thesame time prevent radiation of vits Vibratory `:energy into its support, `a number iof.; magnets 3-3,.134,.35,.36 laremounted on the framework i6 vwith their 4,poles embracing :equallyfspaced portions :ofthe strand l0. Adjacent .magnets of .the fourlhave'their. north poles atopposite sides of the'fstrandso that, .when an'electric impulsevis passed :through thelstrand in one direction, its upperiquarter vand third .quarter areurged inxone direction by ythemagnets 33 and 35, while its secondquarter and fourth-quarter are urgedin theoppositedirection by the magnets 34 and 36 asfshown inEig. 2. The vibration of equal portionsbf the strand in opposite phase serves to eliminate the large transfer of mechanical energy from the strand to the frame which would take .placeif-theyvibrated in the same phase so as 4tof=causefafreactionibetween the strand and its support.

-It mili-,bef noted that the arrangement of magnets which has been described 'results in 4setting -therstrandxinto its'fourthimodefofivibration. .The .fourth Imodeoi vibrationisinot essential, butit :may `befn'oted that when the strand isset into vibration .withA equal portions of the `-wire vibrating'infopposite phase, the yhigher' the mode-of Avvibration used, theveless will be the transferer energy'to the irame. Dynamic balancing `may `beobtained*even in the'lirst mode-of vibration 'bytheuseof adouble strand H11-:in1 whose halves circuit. lAs, Ashown in Fig.

6 electric .impulses :flow :opposite zdirections, as showniin Fig. `3, so ..that .the rtwo halves Vvibrate in opposite phase.

Eilters :incorporating .the :mechanical element whichihas :been described are vshown in Figs. .4,15 andi. -Fig. llshowsan` input circuit @El 'connected to :an outputs-circuit 4| by meansoia balanced networklAZ; .onefo whosearms contains thevibratory :strand all) 4previously described. The balancednetwork `41.2 has the form of a Wheatstone bridge l:Whose v.upper arms contain equal `resistancesiRl and R2. vOne of the lower arms contains a:.resist0r.;R3.and the vibratory strand I0, while the other lower arm of the vbridge containsa resistor'Ri-andastrandr which is precisely like the'str-and l il 'except that it is not in a magnetic iield. '.ilheresistancesof the resistors R4 an'dRs are equal.

A very accurate :balancing ofthe resistances of the bridge circuit may be obtained by making the strand '43 of thesame material'and of .the same length as thestrand `Il) and keeping itat the-same` temperature as'the'strand -It by placing it fwthinithe `insulating casing which has been described. Howeven'diferencesin tension cause slight .changes in the static resistance of the strand l.,'sc`that, for a perfect resistance balance, .the-strandfJlS should be subjected to the same tensionl as the strand Ill. An arrangement foraccomplis'hing this is indicated in Fig. 1 which shows the strand 43 extended between two clamps l lifand lIZr-whiclivare .mounted on the same supports fasfthe .clamps I.I and l2 which hold vthe ends .ofithe strand I0. When the device is constructed, the initial tension on the strand 43 is made the same as that on the strand It, and the two tensions remainequal during tension regulation becauserthe-.endsof thetwo strands are held .by -the :same supports. Electrical connections a, brand c to -the strands l0 and r43 are shown in Fig. l and indicated bythe same 4letters in Fig. 4.-.

As ishown' in Fig.-4, the inputcircuit El is .ccnnected tothelupperfand lower corners of the network 162 whilefthe output circuit is connected `to the sidecorners of thenetwork. As the network is perfectly balanced `when vthe Vibratorystrand IB is not in motion, no current will flow in the outputicircuit'when the strand is still. When, however, -the input circuit receives an alternating current `of a .frequency the same as the natural frequencyof the strand at/the tension to which the strand lill has been adjusted, the strand-li) willbe setinto vibration. The vibration of the strand I0 in the magnetic eld develops therein a counterelectromotive force in well-known manner, thus :developing an effective dynamic .impedance greater thanits static which will unbalance the rbridge and thus allow current of this frequency to pass into the output circuit 4I.

To avoid such-.slight changes inthe frequency of vibration of the strand Ill as may occurwith changes in.` its amplitude of vibration, the amplitude of :vibration of the strandis limited by placing an automatic volume control 134 in the input 4, the automatic volume control .may `take the form shown in U. S. Patent 2,025,775issued to me on December 31, 1935, or it may be of other known type. Its efect is to limit the amperage of the current passed through the strand I 0, thus controlling the forces between the parts of the strand and the magnets 'and so limiting the amplitude of the vibration ofA thestrand. In order that limiting of thevcurlrent'maysecure-this effect, it is importantthat the magnetic field in which the strand is' placed be maintained constant. This result can best be achieved by producing this field by means of permanent magnets as shown in Fig. l.

Sharpness of tuning requires that radiation of energy from the vibrating strand be limited. The means for limiting the radiation of mechanical energy have already been described. Limitation of the electrical radiation from the vibrating strand is attained by making the impedance of the network connecting the two ends of the strand Hl, including the network 42, the input circuit 40 and the output circuit 4l, at least as great as the static resistance of the strand. The network impedance 4between the ends of the strand may most desirably be made greater than this, and, where sharp tuning is important, should be increased until a further increase causes no substantial increase in the Q of the circuit.

Fig. 5 shows a modified filter in which the balanced network 421 consists of a capacity bridge. The operation is similar to the operation previously described, and electrical radiation from the vibratory strand is limited in the same way.

Fig. 6 shows a third form of filter incorporating the same mechanical element. In this case, the input circuit 402 and the output circuit 412 are linked by a transformer T. The balanced network 422 includes two halves of the primary of the transformer T and the strands l0 and 43. The

input circuit is connected to the middle point of the primary coil of the transformer and to a point between the strands I0 and 43. When the network 422 is balanced, the currents through the two halves of the primary are equal and opposite, so that no voltage is induced in the secondary circuit. When, however, the frequency of the incoming current is suoli as to set the strand I into vibration at the frequency to which its tension has been adjusted, the dynamic resistance of the strand unbalances the circuit 422, allowing more current to flow through one-half of the primary than through the other half so that a voltage is induced in the secondary circuit. Thus, the transfer of energy between the circuits occurs as .before only when the frequency of the incoming current is that to which the tension of the strand has been adjusted.

Any one of the filters which has been described may be converted into an oscillator by connecting the output circuit to an amplifier' and providing y a feedback to the input circuit. Fig. 7 shows an oscillator containing the lter circuit shown in Fig. 3, except that a different form of automatic volume control is used.

In the oscillator shown in Fig. '7, the output circuit 4l of balanced network 42 is coupled through a transformer Ti to an amplifier A, and a portion of the plate energy from the tube of the amplifier is fed back into the input circuit 40, In this case, control energy for the automatic volume control is taken from the plate circuit of the ampliiier passed through an automatic-volume-control rectier 44 of conventional design and connected to control the grid bias which determines the gain of the amplifier. The output from the amplifier A is led through a transformer T2 to the point at which the energy from the oscillator is to be used.

What I claim is:

' l. A precision frequency-determining unit adjustable over a wide range of frequencies comprising a wire of drawn tungsten and a stretcher for said wire comprising a bar of glass-bonded mica greater in length than the wire having one of its ends secured to one end of the Wire, and

a bar of aluminum connecting the other end of the mica bar to the other end of the tungsten wire, whereby thermal expansion of the wire is substantially balanced by that of the stretcher; and means for adjusting the spacing of said mica bar and said aluminum bar positively to deform said wire longitudinally, thereby to adjust the natural frequency thereof.

2. A precision frequency-determining unit adjustable over a wide range of frequencies comprising: a pair of spaced relatively adjustablyfixed rigid supports; a tensioned elastic straight conductive strand extending between and rigidly secured to said supports; means for developing a magnetic field transverse to said strand; a supply circuit connected to said strand for impressing thereon signals of a frequency related to the natural frequency of said strand; the eld distribution of said magnetic iield being such that equal portions of said strand vibrate in mechanical phase opposition; and means for adjusting the spacing of said supports positively to deform said strand longitudinally, thereby to adjust the natural frequency thereof.

3. A precision frequency-determining unit adjustable over a wide range of frequencies comprising: a pair of spaced relatively adjustablyfixed rigid supports; a tensioned elastic straight conductive strand extending between and rigidly secured to said supports; means for developing a magnetic field transverse to said strand; a

supply circuit connected to said strand for im- 'said supply circuit and said output circuit except that due to vibration of said strand; and means for adjusting the spacing of said supports positively to deform said strand longitudinally, thereby to adjust the natural frequency thereof.

4. A precision frequency-determining unit comprising: a pair of spaced rigid supports; a tensioned elastic straight conductive strand extending between and rigidly secured to said supports; means for developing a magnetic eld transverse to said strand of such a field distribution that equal portions of said strand vibrate in mechanical phase opposition; and a supply circuit connected to said strand for impressing thereon signals of a frequency related to the natural frequency of said strand.

5. A precision frequency-determining unit comprising: a pair of spaced rigid supports; a tensioned elastic straight conductive strand extending between and rigidly secured to said supports; a plurality of magnets of alternating polarity disposed along the length yof said strand for causing equal portions of said strand to vibrate in mechanical phase opposition; and a supply circuit connected to said strand for impressing thereon signals of a frequency related to the natural frequency of said strand.

6. A precision frequency-determining unit comprising: a pair `of spaced rigid supports and a supporting structure extending therebetween; a tensioned elastic straight conductive strand extending between and rigidly secured to said supports; means for developing a magnetic field transverse to said strand of such a field distribution that equal portions of said strand vibrate in mechanical phase opposition; and circuit terminals connected to said strand for translating thereto signals of a frequency related to the natural frequency of said strand, said supports and supporting structure being constructed of parts substantially free of local strains.

7. A precision frequency-determining system comprising: a pair of spaced rigid supports; a tensioned elastic straight conductive strand extending between and rigidly secured to said supports; means for developing a magnetic eld transverse to said strand; a supply circuit connected to said strand for impressing thereon signals of a frequency related to the natural frequency of said strand; an output circuit for deriving signals from said strand; and an electrical impedance network interconnecting said supply circuit, said output circuit and said strand and proportioned effectively to eliminate the transfer of electrical energy between said supply circuit and said output circuit except that due to vibration of said strand.

A precision frequency-determining system comprising; a pair of spaced rigid supports; a tensioned elastic straight conductive strand extending between and rigidly secured to said supports; means for developing a magnetic field transverse to said strand; an electrical network comprising a bridge circuit including said strand in one arm thereof; a supply circuit connected across one diagonal of said bridge circuit for impressing thereon signals of a frequency related to the natural frequency of said strand; and an output circuit connected across the other diagonal of said bridge circuit for deriving signals from said strand.

9. A precision frequency-determining system comprising: a pair of spaced rigid supports; a pair of tensioned elastic straight conductive strands extending between and rigidly secured to said supports; means for developing a magnetic field transverse to said strands; an electrical network comprising a bridge circuit individually including said strands in two arms thereof; a supply circuit connected across one diagonal of said bridge circuit for impressing thereon signals of a frequency related to the natural frequency of said strands; and an output circuit connected across the other diagonal of said bridge circuit for deriving signals from said strands.

10. A precision frequency-determining system comprising: a pair of spaced rigid supports; a pair of tensioned elastic straight conductive strands extending between and rigidly secured to said supports; means for developing a magnetic field transverse to said strands; an electrical network comprising a bridge circuit individually including said strands in adjacent arms thereof; a supply circuit connected across one diagonal of said bridge circuit for impressing thereon signals of a frequency related to the natural frequency of said strands; and an output circuit connected across the other diagonal of said bridge circuit for deriving signals from said strands.

11. A precision frequency-determining system comprising: a pair of spaced rigid supports; a pair of tensioned elastic straight conductive strands maintained at the same temperature extending between and rigidly secured to said supports; means for developing a magnetic eld transverse to said strands; an electrical network comprising a bridge circuit individually including said strands in two arms thereof; a supply circuit connected across one diagonal of said bridge circuit for impressing thereon signals of a frequency related to the natural frequency of said strands; and an output circuit connected across the other diagonal of said bridge circuit for deriving signals from said strands.

12. A precision frequency-determining system comprising: a pair of spaced rigid supports; a pair of tensioned elastic straight conductive strands maintained under the same tension extendingv between and rigidly secured to said supports; means for developing a magnetic eld transverse to said strands; an electrical network comprising a bridge circuit individually including said strands in two arms thereof; a supply circuit connected across one diagonal of said bridge circuit for irnpressing thereon signals of a frequency related to the natural frequency of said strands; and an output circuit connected across the other diagonal of said bridge circuit for deriving signals from said strands.

13. A precision frequency-determining system comprising: a pair of spaced rigid supports; a tensioned elastic straight conductive strand extending between and rigidly secured to said supports; means for developing a magnetic field transverse to said strand; an electrical network comprising a Wheatstone resistance bridge circuit including said strand in one arm thereof; a supply circuit connected across one diagonal of said bridge circuit for impressing thereon signals of a frequency related to the natural frequency of said strand; and an output circuit connected across the other diagonal of said bridge circuit for deriving signals from said strand.

14. A precision frequency-determining system comprising: a pair of spaced rigid supports; a tensioned elastic straight conductive strand extending between and rigidly secured to said supports; means for developing a magnetic field transverse to said strand; an electrical network comprising a bridge circuit including said strand in one arm thereof; the static resistance of said strand being substantially less than the impedance of any adjacent arm of said bridge circuit; a supply circuit connected across one diagonal of said bridge circuit for impressing thereon signals of a frequency related to the natural frequency of said strand; and an output circuit connected across the other diagonal of said bridge circuit for deriving signals from said strand.

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